This invention relates generally to metals and metal alloys used in high temperature applications. More particularly, the invention relates to methods and devices for incorporating elemental components in gaseous form into metal compositions, to enhance the properties thereof.
A variety of metals and metal alloys are especially useful for high temperature equipment, e.g., engines and other machinery. As one example, superalloys are the materials of choice for turbine engine components, such as turbine buckets, nozzles, blades, and rotors. The superalloys are often based on nickel, although some are based on cobalt, or combinations of nickel and cobalt. These materials provide the chemical and physical properties required for turbine operating conditions, i.e., high temperature, high stress, and high pressure. As an illustration, an airfoil for a modern jet engine can reach temperatures as high as about 1100° C., which is about 80-85% of the melting temperature of most nickel-based superalloys.
The nickel-based superalloys continue to be tremendously popular because of their high level of performance. However, research efforts in recent years have also focused on alternative materials for high temperature components, such as the turbine engines. Examples of the alternative materials are various refractory metal intermetallic composite (RMIC) materials. Many of these are based on silicon (Si) and at least one of niobium (Nb) and molybdenum (Mo). For example, niobium-based RMIC's often include silicon, titanium (Ti), hafnium (Hf), chromium (Cr), and aluminum (Al).
RMIC materials are described in various references, such as U.S. Pat. No. 5,932,033 (Jackson and Bewlay); U.S. Pat. No. 5,942,055 (Jackson and Bewlay); and U.S. Pat. No. 6,419,765 (Jackson, Bewlay, and Zhao). Many of the RMIC's have melting temperatures of about 1700° C. This characteristic makes such materials very promising for potential use in applications in which the temperatures exceed the current service limit of nickel-based superalloys. Many RMIC's also possess various other attributes, e.g., relatively low density, as compared to nickel superalloys.
The RMIC composites usually have a multi-phase microstructure. For example, the microstructure may comprise a metallic Nb-base phase and an intermetallic metal silicide phase. As described in U.S. Pat. No. 5,833,773 (Bewlay and Jackson), the metal silicide phase sometimes includes an M3Si silicide and an M5Si3 silicide, where M is Nb, Ti or Hf. The materials are considered to be composites that combine high-strength, low-toughness silicides with a lower-strength, higher-toughness Nb-based metallic phase. Some of the RMIC composites include other phases as well. For example, they may further include a chromium-based Laves-type phase modified with silicon. Such a phase promotes oxidation resistance, and useful materials of this type are described in some of the patents mentioned above. The composites are often formed in situ by directional-solidification of the alloy.
The selection of phases and element constituents in the RMIC's is aimed at achieving a balance of properties which are important for a particular end use application. An example of the properties alluded to above include strength (fracture strength and rupture strength), toughness, density, oxidation resistance, and creep resistance. In the case of turbine engines, the appropriate balance of properties is greatly influenced by the ever-increasing need to achieve higher operating temperatures for the component.
Thus, there continues to be considerable interest in even greater improvements in RMIC properties—especially at both high and low temperatures. This need is pronounced in the case of gas turbines, which often cycle between room temperature and 1100° C., as discussed above. New RMIC materials would therefore be welcome in the art. Furthermore, new techniques for making the improved RMIC materials would also be of great interest to practitioners in the art.